Those in the high precision machine tool industry are continually searching for better ways of detecting and compensating for errors in the production of parts in the ranges of thousandths and millionths of an inch. Machines are known which automatically compensate for the wear of the cutting tools themselves, for minor deflections of the cutting tools under pressure that introduce errors, and for so-called "thermal growth" of one part of a machine tool relative to another part of the machine tool.
The detection of thermal growth and the compensation therefor has heretofore been somewhat of an art, given that thermal growth is a less predictable error than other types of machining errors, such as tool wear. Past efforts at compensating for thermal growth include the warming up of a machine tool prior to use, so that the majority of the thermal growth occurs prior to using a machine tool for producing parts, and the continuous running of a machine tool in the hope that a constant, stabilized running temperature had been reached. However, it has been found that there is no such thing as a perfect stabilized running temperature, owing to the vagaries of individual machine tools, and to the fluctuations of the environmental temperature of the area in which the machine tool is used. Furthermore, it is time consuming, wearing on machine components, and expensive to idle a machine tool for thirty minutes to an hour prior to use.
An early device for temperature compensation is disclosed in U.S. Pat. No. 2,956,344 to Rantsch, which device required two separate, spaced apart rods exhibiting small thermal expansion, each rod having measuring marks, and a device for detecting the relative displacement of the measuring marks.
In U.S. Pat. No. 2,664,787, a control rod of steel, invar metal, quartz or other material having minimum temperature sensitivity as respects expansion or contraction under normal operating conditions was essentially used as a lever for actuating switches to energize a motor in one of two rotational directions in order to compensate for thermal growth.
U.S. Pat. No. 4,530,626 to Sabbioni discloses a bar made of material of a very low coefficient of thermal expansion that uses a scale or reference element in its detection device so as to measure the distance from a tool to a piece of material to be cut in a gear cutter.
A recent high production machining device is disclosed in U.S. Pat. No. 5,005,452 to Wood, III, in which each of two slides of a machine tool has an associated rod of material such as In-var.TM. (Ryerson Steel of Cincinnati, Ohio) having a coefficient of linear expansion of only 0.000001 per unit length per .degree.C. An etched glass slide is attached to a reference surface of an upper one of the slides, an optical reading head is utilized to provide a linear position signal to the machine's control unit, and the thermally inert rod of In-var.TM. is attached to the other one of the slides. One or both of the slides has a linear scale on the low thermal expansion material attached to the reference surface.